FIG. 1 shows a circuit breaker 10 which feeds power on a line 12 to a machine, apparatus or other load 14, from a bus 16. A current transformer 18 scales down the power system current, referred to as primary current, so that a protective relay 20 can safely measure the current. This scaled down current is referred to as secondary current herein. If the magnitude of the secondary current from current transformer 18 is above the setting of the protective relay 20, as it will be if there is a fault on the line, then the protective relay will "pick up" and trip the circuit breaker 10. The circuit breaker 10 interrupts the primary current. As a result, the secondary current in the transformer stops flowing, and the protective relay eventually drops out.
The safe operation of the power system serviced by the protective relay frequently depends on a rapid and accurate determination that interruption of primary current has occurred. For example, if the circuit breaker 10 fails to interrupt the primary current (as might happen if the breaker mechanism sticks, or a successful arc interruption does not take place) then other circuit breakers closer to the sources of electric power feeding the bus 16 must quickly operate. Further, even when there is a normal interruption, rapid and accurate confirmation that the breaker has in fact opened is often required.
Secondary current (or lack thereof) is often used to rapidly determine that the primary current is zero, which in turn is an accurate indication that the breaker has in fact opened. Such a determination of breaker status using secondary current is affected by both the design of the relay and also by the performance of the current transformer.
Referring to FIG. 2, almost all current transformers consist of an iron core 24 with a secondary winding 26. The primary of current transformer 18 is typically a conductor 28 which makes one pass through core 24. Power system current flowing in the primary winding of the current transformer 18 produces secondary current at the secondary winding 26. The secondary current is applied to a protective relay 20. The secondary current is further scaled down by a protective relay current transformer 32. The output of current transformer 32 is connected to an analog low pass filter (LPF) 34, the output of which is applied to an analog-to-digital converter (ADC) 36. The digital samples from ADC 36, referred to hereinafter as raw current (RAW I), are applied to a digital filter 38. The output of digital filter 38 is referred to hereinafter as filtered current (FILTER I). The functions enclosed by the dotted line are performed by a microprocessor.
The magnitude of the filtered current is then determined by magnitude calculator 40 and the result is applied to a comparator 41, which compares the magnitude against an overcurrent threshold at input 21. If the magnitude is greater than the overcurrent threshold then an overcurrent element 22 is picked up. If the magnitude is less than the overcurrent threshold at 21 then overcurrent element 22 is not picked up or drops out after having been previously picked up.
FIG. 3 shows a model of current transformer 18 and the connected burden (which includes the input impedance of protective relay 20) with the circuit breaker closed. FIG. 3 also shows the direction of the secondary current flow, which comprises current through the transformer excitation branch 42 and the burden impedance 44 when the power system current is flowing. Those two currents are in-phase with the power system current.
FIG. 4 shows the secondary current flow when the power system current source is not present, i.e. circuit breaker 10 has opened up. At that point there is no longer an AC component to the secondary current, only a decaying exponential current. The current throughout the burden impedance flows opposite to the direction of current flow when the breaker was closed.
FIG. 5 shows scaled power system primary current and secondary current of the current transformer 18 during and immediately following a fault. In FIG. 5, the power system primary current is successfully interrupted by the circuit breaker 10 at a time "t". Ideal secondary current is shown at 15, while actual secondary current is shown at 17. The secondary current, however, is not immediately interrupted in many current transformers. At time t, the magnetic flux in the current transformer core is very high. The flux decays from this high value over time. The decaying flux results in a decaying unipolar current flowing in the secondary of current transformer 18. The decaying current is characterized by no zero crossings and eventually subsides to zero over an extended period of time. This decaying current in the transformer secondary is referred to as subsidence current.
Subsidence current can cause the overcurrent element of the protective relay to remain picked up (indicating erroneously that primary current is still flowing), long after the circuit breaker has, in fact, actually successfully interrupted the primary current. This extended pick up of overcurrent elements is called delayed drop out. If an overcurrent element is used to indicate breaker status and has a very long drop out delay then the control circuits of the power system might improperly make a serious system error indicating that the primary current has not been interrupted and then initiating remedial and undesirable action, such as tripping additional breakers.
The subsidence current in the secondary of the current transformer creates the same problems regardless of the characteristics of the protective relay, i.e. whether the relay elements are electromechanical in nature or in a microprocessor implementation in which successive samples of secondary current are processed with computer algorithms. Subsidence current is neither DC nor a linear ramp signal but instead is a decaying exponential. Accordingly, the filtering methods for the protective relays cannot fully reject the decaying exponential subsidence current, which in turn keeps the overcurrent elements that are set to a low threshold asserted. Hence, the problem with subsidence current remains, even with the modern computer relays.
A primary purpose of this invention is to reduce the time it takes to determine that the primary current in the circuit breaker has been successfully interrupted. In the present invention, the presence of subsidence current is identified and appropriate action taken accordingly.